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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 31st May 2020, 11:25 pm   #1
Sparky67
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Default 12V 20A PSU Crowbar Cct

Hi all,

I have been looking through various websites for a cct with component values for an overvolts SCR crowbar (operating at say 15V) for a typical 12V 20A analogue ham radio PSU. There seems to be lots of discussion about them but so far I haven't found anything I can build and incorporate into my PSUs. Does anyone have a suitable design please?

Many thanks.

Martin
G4NCE
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Old 1st Jun 2020, 12:00 am   #2
Bazz4CQJ
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Default Re: 12V 20A PSU Crowbar Cct

I built a crowbar external attachment to use with a commercial linear PSU which I use for my ICOM IC718, circuit attached. Sorry it's tatty, not intended for publication.

There's a Transil diode for fast protection. The two zener diodes (ZD) in series were just hand-picked to add up to the voltage that I wanted to trip at. Seems to do the job.

B
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Old 3rd Jun 2020, 12:06 am   #3
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Default Re: 12V 20A PSU Crowbar Cct

Thanks Barrie, I will look at building that design. From a quick bit of research I did it looks like a blade fuse may the way to go to carry the required current OK and to open quickly to protect the external device when the SCR triggers.
Cheers.
Martin
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Old 3rd Jun 2020, 8:31 am   #4
Radio Wrangler
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Default Re: 12V 20A PSU Crowbar Cct

That should do the job, but you probably should try it out somehow, just to feel confident it will save your bacon if it ever has to.

Motorola did a crowbar trigger IC that you programmed with the ratio of two resistor values, but I can't remember the type number. It's academic if they aren't available anyway.

If you're designing a complete power supply, you can integrate the crowbar into it in ways that make it a lot less dramatic. In the one I did, I have the crowbar do two things. It opens the relay used for mains on/off, and it uses its thyristor to dump the charge in the reservoir capacitors directly. If I trigger it, there is a little click and I hear the mains switch relay release. It does mean that this supply needs a push on a start button to turn it on, but the whole lot integrates with a soft-start arrangement. When I next come across the circuit diagrams, I'll post them in this group.

David
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Old 3rd Jun 2020, 9:33 am   #5
turretslug
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Default Re: 12V 20A PSU Crowbar Cct

The Motorola crowbar IC was probably the MC3423- I acquired a few from a big work-place clear-out long ago (wasn't the inefficiency and management change in British industry occasionally useful, ill wind and all that....) As David says, very simple to make a safety crowbar and if you want further sophistication, the application note is your oyster. It can be made into a postage-stamp matrix-board module light enough to be mounted by its thyristor tab- small enough for a 16V one to go inside my FRG-100 (very low down on the true enthusiast's pecking-order of radios, but expensive and useful enough to me!) and a 19V one inside the TF2002B (worryingly generous regulator headroom, besides I'd sorted out about a dozen different faults on acquisition and didn't want to spend any more time than I could avoid in its innards!)

Colin
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Old 3rd Jun 2020, 9:45 am   #6
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Default Re: 12V 20A PSU Crowbar Cct

That would be the MC3423 crowbar IC. It senses the output voltage and if over the limit triggers a thyristor to short the output. However you need a thyristor with a suitable current rating to discharge the reservoir capacitors.

However it's all academic as David says as the IC is now obsolete.

Keith
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Old 3rd Jun 2020, 10:03 am   #7
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Default Re: 12V 20A PSU Crowbar Cct

Plan A is to build and test Barrie's suggested cct on my dual-output bench PSU and then incorporate it into the first (cheapest!) 20A PSU and soak-test it there for a while with the normal levels of on/off and Tx/Rx switching and RF etc in the shack.

I am also considering incorporating a relay soft-start cct into both PSUs, or maybe a 100A pre-built current limiter available from China, or whether a simple NTC thermistor would do the job...?

Martin
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Old 3rd Jun 2020, 11:54 am   #8
Steve G4WCS
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Default Re: 12V 20A PSU Crowbar Cct

the PW marchwood was regarded as a decent design and the article describes the various protection circuits used. Practical wireless June 1983 ( scanned in americanradiohistory)

I have PDF copies but they are too large to upload, sorry
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Old 3rd Jun 2020, 1:29 pm   #9
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Default Re: 12V 20A PSU Crowbar Cct

I built the Marchwood PSU and it worked well, with soft start and crowbar, can't believe it was 37 years ago. Ted
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Old 3rd Jun 2020, 1:45 pm   #10
Steve G4WCS
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Default Re: 12V 20A PSU Crowbar Cct

another useful article here .

http://warc.org.uk/?page_id=404
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Old 3rd Jun 2020, 6:10 pm   #11
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Default Re: 12V 20A PSU Crowbar Cct

Quote:
Originally Posted by Steve G4WCS View Post
another useful article here .

http://warc.org.uk/?page_id=404
I thought I recognised this. Many (many) years ago when I was in my first job, one of the other guys in the office was into amateur radio, but wasn't really into electronics construction. He had tried to build this design, but it didn't work. He asked me to look at it and after a bit of work (poorly soldered connections mostly) I got it working for him.

Some years later I inherited a rather dead heavy duty BNOS power supply with a pair of nice torroidals, a big case and space to mount a decent number of 2N3055s. I dug out the photocopy of the article from AR magazine he had given me to help fix his, and built it up. It can supply more than the 20A of the original design, and has adjustable voltage and current limit. And it's still working! Have to admit I've been thinking of getting rid of it, as it's so heavy and I have a nice Farnell AP 60-50 that's smaller, lighter and more powerful (as it's a switcher).

EDIT: oh and I've also found that original photocopy from more than 30 years ago!
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